Field of the Invention and Related Art Statement
The present invention relates to an image recording appartus of heat generation type, and more particularly to an image recording apparatus of heat generation type in which a background level can be adjusted and a gray level can be reproduced in a precise manner by means of a simple circuit construction.
There has been practically used an image recording apparatus of heat generation type comprising a thermal head having a number of heat generating resistors arranged in a line and a driving circuit for supplying electric currents to respective resistors in accordance with an image to be recorded. In general, the heat generation type image recording apparatus may be classified into a heat sensitive type recording apparatus which utilizes a heat sensitive record paper having a substrate and a heat sensitive coloring material layer applied on the substrate, and a heat transfer type recording apparatus comprising a thermal head and an ink ribbon, an ink on the ribbon being thermally transferred onto a record paper. In these heat generation type recording apparatuses it is required to set or adjust a background level, i.e. a density of a background area as well as to reproduce the tone in a correct manner in accordance with an image to be recorded.
In the heat generation type image recording apparatus, there has been proposed to produce a number of record pulses having different pulse widths corresponding to gray levels of an image to be recorded and the record pulses thus formed are supplied to respective heat generating resistors of a thermal head. In such a known recording apparatus, the driving circuit has to produce various kinds of record pulses the number of which corresponds to that of the gray levels, so that the construction of the driving circuit becomes very complicated. Therefore, in the known device, there is provided only one driving circuit, and the record pulses produced by the common driving circuit are successively supplied to respective resistors via a switching circuit. Then, the recording speed becomes low.
It has been further proposed to change an amplitude of the heat generating current in accordance with gray levels. Such a method is called the area-tone method. It is also known to use the combination of the record pulse width modulation method and the area-tone method.
In the known recording apparatus, a pulse duration of each record pulse is determined by the preheating energy, the background level and the gray level. Therefore, when the background level is to be adjusted, it would be necessary to change the whole pulse width accordingly.
In the above mentioned known methods, if it is required to reproduce 32 gray levels, it is necessary to produce 32 different kinds of record pulses having different pulse widths and/or amplitudes, so that the driving circuit is liable to be complicated in construction. Further, if it is also required to change the background level, the widths of the record pulses have to be varied accordingly. For instance, if there must be provided five background levels, it would be necessary to produce 160(=32.times.5 ) record pulses having different pulse widths. Therefore, the driving circuit becomes further complicated. Therefore, if a number of heat generating resistors of the thermal head are energized parallelly by means of a number of driving circuits, the whole circuit would become extremely complicated. In order to mitigate the above drawback, it has been proposed to supply driving pulses to successive heat generating resistors serially from a single driving circuit. Then, a longer time is required to effect the recording of one dot line and the recording speed becomes lower.
Moreover, when an amount of generated heat is controlled by changing a pulse width of the record pulse, it is difficult to obtain stably the record image having good tonal property due to various factors such as a threshold value for coloring the heat sensitive record paper and a transferring threshold value for transferring an ink onto the record paper. Further, in the area-tone method, it is quite difficult to control a size of a coloring dot accurately and thus a resolution of the recorded image becomes worse.